High efficiency blue-emitting zinc cadmium sulfo-selenide phosphor



Patented July 26, 1949 HIGH EFFICIENCY BLUEFTEMITTINGZINCoADMIUMfsULFo-sEnENrDEPHosBHoR xlxHumboldtw;IiBVEFenz,Princetonymal'aassignor .-.to:Ra'dio .ConporatiomohAmeriea, ar-corporation of Delaware Applicationzlu'ne 29, 1946; S'l'iifNOFGSEWZ '5'Claims (C|z252.3.01i6) 11 .This invention relates to improvements, ininorganic crystalline materials which are visibly luminescent.whenexcited'by energy within a certainsband of wavelengths. 1 Moreparticularly, it

-relateswtorimprovements in inorganic phosphors,

materialshaving peak emission within a very restricted portion of thevisible spectrum.

Heretofore, the best luminescent materialshaving peak emissions in theblue range of about 4*;400rto 4300K. "have-beenthose'consisting of memor u a irphotomulfipli "well known zinc and cadmium sulfides activatedTwithsilver. .These previously known "phosphors have relatively highluminescence'before processing but after beingsubjected to the"operations Still another-object is'to-provid'e a blue=emittingphosp'h-or which' carr' be combirie'd with acomple- -menta-ry,efiicient; yellow emitting- (a greenfernitting=- plus red emitti-ngyphosphor (s) to pronecessary to incorporate theni'into a screenin a*'duce-- aphosphor ha ing increased e'ificiency-cf L'There'have now beendiscovered a new group of s phosphor compositions having very highpeak'-'-white=ernission" useful 'in television or for genral illumination.

"' 'These and other-'nbjectswillf homers-apparent and the invention-vvi1lb'e better understood from emission in the blue range.i'Thesephosphors re-" the"desoriptionwhich followsincluding'whe'acftaina high efficiency, even'after'processing, and remain stable over.prolonged periods." These compositions contain theions, zinc",sulfunand selenium, r and usually, although not necessarily, Y

*companying drawings ofwhich,

Fig. 1 is a geometrical representationof a system of chemitzalcompositions "containing" zinc, '"cadrnium; "sulfur, *and selenium,"showing a" plane cadmium. They are activated'only with silveri definingthat particular part ofthe "system'in sPhosphorcompositions containingthe fourfba'sic .jions vand either with or without silver: or copper.asactivator's have beendescribed in a co=pending ap"p1ication',.SerialNo. 678,589fflledllun22f.I946,

which the four ions are present in stoichiometrical proportions.

-...Fig.1,2si a geometrical representation of the :compo'sitions"of the'present" invention which con- .and assigned to the present'assig'nee.jAlthough tainzinc, cadmium, sulfur, and selenium within .containing' atleast three-and sometimes allot the ,sameiiou'r', basic ions, zinc;cadmiumjsulfuriand {selenium} the. materials embodied in thei'present'....inventioni*fall outside the range of "compositions ijdisclos'ed' inthe said application since those comthe limits of. theintersectingrplaneiof Fig. 1, plus 1 an added -a'ctivator. Theisbasehorizontal,plane I- of this figure is-the rectangularmlane showninFigure 1.

,ReferringtoEigure, l, the tetrahedron l representsiall "possible'hhemical' combinations of the ielements, :ZnTCd; Sian'd seliTheiplane 2 which .cutssacrosfthis.tetrahedron and. has intercepts zns';LC'dS, C689,, a'ri'dTZiiSeis .meant to represent 'lfil g v(Juisidt?thePrescribed-"limits, 'eitheliifdid 40.fallco'xnbina'tionsin.whichthejrfour basic elements notqemitat all orhad 'efiicienc'ies. so low as not" to ".Lbe'sofhpra'ctical value.However, it'hasbeen'unexpecte'dly; foundthat a certain other" range 'of"compositions o'f i-the four ions and'sometimesilof .are lpresent linvstoichiometric proportions. In .jother. words msazessse only, three" ofthem emits very stronglyirithei'blue "fon agram' atomic'basis. Thisisthefirstlimita- "'tion' on the compositions-of the present inventionsimilarlywo those compositions defined in the v abovementioned-expendingapplication.

tainingZn,Cd*,'S,"an'd Se ions which emit strong- The compositions ofthepresentinventi'on con- :lyiin theiblue region ofthespe'ctrum.

Another ob'ject'is to provide phosphorcomp'osi- '"jtions having Epeaki'emission between "'4'gfl0'tiand 7315800 A: which'j'areihighly stable.

taintvarying.smallfamountsof silver as an acti- ':vator.,IntgenaraL't'the activator may be'present .in amounts rangingffrom"T020001;- to 0205fpercent 'iibnwe'ig'ht' oifiith'e 'tptarjamounticf'pnosphorimatef Another object is to'providephosphorcompbsi aerial.n'preferfewamount'is'about"0.005percent origin 0, to l zinc selenide andno zinc sulfide at the opposite end of the axis which is not shown.Since the stoichiometric proportion of zinc remains constant along thisaxis, it may also be said to represent compositions in which the molarratio of sulfur and selenium varies in the same manner. origin thereforerepresents a composition having a molar ratio of .9 sulfur to 0.1selenium. The proportions along this axis are expressed as decimals withthe entire length of the axis taken as unity and each division being0.1.

Similarly, the Y-axis represents various proportions of zinc and cadmiumsulfides and the Z-axis represents various amounts of silver added as anactivator up to 0.05 percent by weight of the total base materialphosphor content.

The high efliciency com-positions of the present invention arerepresented by the smaller cuboid 3 occupying one corner of the largecuboid. This figure illustrates several characteristics of thecompositions. One of these is that compositions may be made up whichcontain no cadmium. These would be all the compositions falling withinthe front plane face of the small suboid 3. Other characteristics arethat the compositions contain molar ratios of from .86 to .998 sulfurcompared with from .002 to .14 selenium, and from .95 to 1 zinc,compared with zero to .05 cadmium, and .0001 to .05 percent by weight,based on the total weight of phosphor base material, of silver asactivator.

Some specific examples of compositions made according to the presentinvention are shown in The anhydrous pure salt-s are comminuted and wellmixed in a fused-silica crucible, moistened with pure distilled waterand the activator solution added. The mixture is stirred, evaporated todryness and heated in a silicon carbide resistance furnace at or aboveatmospheric pressure, starting at room temperature and attaining 780 C.in about 45 minutes. The temperature is held at this point for 6 minutesafter which the reaction product is removed from the furnace and cooled.Conditions are provided during heating and cooling to allow only aminimum amount of oxidation. One effective way of doing this is to placethe crucible containing the phosphor mixture within a larger coveredcrucible packed with zinc sulfide. leaking into the larger crucibleunites with the packing of zinc sulfide and therefore cannot adverselyaffect the substances within the smaller crucible. The sodium chlorideserves as a flux.

The resultant phosphors are washed to remove soluble chlorides andsodium sulphide and selenide produced by reaction between the fiux andphosphor ingredients. Alkali halides other than sodium chloride are alsopreferred as fluxes. Their concentrations in the initial furnace batchmay be varied from 0.01 to percent by weight although optimum resultsare obtained in the range of about 0.5 to 4 percent.

The crystallization temperatures and heating and cooling times are notvery critical. The temperature of crystallization may be varied fromabout 500 C. to about 1,400 C. and the The first division to the rightof the 7 Any oxygen crystallization time may be varied from about 5minutes to about 20 hours depending upon the particle size desired inthe final phosphor.

When the phosphors listed in Table l are crystallized at 780 0., theyhave the following characteristics under cathode-ray excitation:

The figures of column 2 above are the relative energy outputs comparedto the output of a standard consisting of alpha-zinc silicate activatedwith manganese (a-Zn2SiO4:Mn). The luminescence of the standard has beentaken as equal to units. The figures in column 3 are the relativeluminescence outputs as detected by the normal human eye. The eye doesnot have peak sensitivity at the same point as certain objectivemeasuring instruments nor does it respond to radiations outside thevisible band of the spectrum. For these reasons, it happens that thetotal luminescence detected by the eye is considerably less than thatdetected by a bluesensitive objective measuring instrument in the caseof the improved phosphors of the present invention.

The relative peak efiiciencies of phosphors A, B, and C are 20 to 50percent higher than the previously most eflicient ZnSzCdSzAg phosphorswith corresponding peak wavelengths.

By mechanically mixing the superior blueemitting phosphors of thepresent invention with complementary optimum-efficiency, yellow-emittingphosphors such as one containing from 45 to 55 weight percent zincsulfide and 55 to 45 weight percent cadmium sulfide and activated withsilver, or with the yellow-emitting phosphors described in thepreviously mentioned co-pending application, a phosphor is producedhaving improved efficiency of white light emission. This greaterefiiciency is present under cathode-ray excitation, which makes itValuable for television purposes. and under corpuscular, or undulatoryexcitation such as ultraviolet, X-rays, etc., which also makes it ofvalue for general illumination purposes.

1 The selenium content in these new phosphors affords greater stabilityduring application and operation and results in favorable decrease inabsorption of visible light as compared with the nonselenium-containingZnS CdS Ag phosphors.

The new phosphors are further useful for transforming invisibleradiations such as alpha particles, gamma rays, and other corpuscular orundulatory radiations into blue light which is particularly favorablefor actuating blue-sensitive photoresponsive devices such asphotomultipliers.

I claim as my invention:

1. A luminescent composition consisting essentially of the reactionproduct formed by heating together, at from 500-1400 C. for from about 5minutes to about 20 hours in the presence of a 1 flux, substancesconsisting of at least the first three of the four basic elements,sulfur, selenium, zinc, and cadmium, in the presence of from 0.0001 to0.05 per cent by weight of silver as an activator and in which the basicelements are present in the following molar ratios: sulfur.86 to .998,compared with selenium.002 to .14, and zinc-.95 to 1, compared withcadmium'0 to .05.

2. The composition of claim 1 in which the silver activator is presentin an amount substantially equal to 0.005 percent by weight of the totalphosphor base material.

3. A luminescent composition consisting essentially of the reactionproduct formed by heating together, at from 500-1400 C. for about 5minutes to about 20 hours, substances consisting of at least the firstthree of the four basic elements, sulfur,

selenium, zinc and cadmium, in the presence of 0.0001 to 0.05 percent byweight of silver as an activator and from 0.01 to 20 percent by weightof an alkali halide flux and in which the basic elements are present inthe following molar ratios: sulfur- 0.86-0.998, compared withselenium-0.002 to 0.14, and zinc-0.95 to 1.0 compared with cadmium 0 to0.05.

4. A composition according to claim 3 in which said flux is an alkalichloride.

5. A composition according to claim 4 in which said alkali chloride ispresent in an amount of about 0.5 to 4.0 percent by weight.

HUMBOLDT W. LEVERENZ.

REFERENCES CITED UNITED STATES PATENTS Name Date Leverenz June 25, 1946Number

